1. Field of the Invention
The invention relates in general to the monitoring of refrigerated containers, and more specifically to monitoring refrigerated containers over electrical distribution power lines.
2. Description of the Prior Art
Electrical utilities have used their high voltage transmission lines for many years for communication with remote switching and substation sites, for supervisory control purposes. Transmission lines are ideal for communication as they extend from the power generation site to the remote sites without intervening obstacles. Use of the distribution power lines, however, has been slower to develop, as the distribution power lines are susceptible to electrical noise and interference, and they include distribution transformers, electrical loads, sectionalizing switches, capacitor banks, and the like, which attenuate communication frequencies. The increasing desirability of being able to selectively and remotely control electrical loads on the distribution system, and the availability of low cost encoders for automatic meter reading, have produced a flurry of activity in the use of distribution power lines for communication purposes. U.S. Pat. Nos. 3,911,415; 3,942,168; 3,942,170; 3,967,264; and 3,980,954, describe some of the early problems encountered, and solutions thereto, when using electrical distribution power lines for communication.
A specialized use of electrical distribution power lines for communication purposes has been disclosed in U.S. Pat. No. 4,234,926. Refrigerated containers, called "reefers" are monitored by a central computer, using the power lines for polling or interrogating remotely located computerized monitoring units associated with the reefers, as well as for receiving reefer status data in return. Since large numbers of reefers are stacked aboard ships, as well as in ship terminals, the use of the electrical power lines connected to the reefers for the additional functions of monitoring and control substantially increases the speed and reliability of the monitoring process, which is normally manually performed.
While the use of ship and ship terminal electrical distribution lines for communication may seem simple and straight forward compared with the attenuating obstacles encountered by the electric utilities in using their distribution systems for communication, ship and ship terminal electrical systems can have substantial amounts of electrical noise. For example, large adjustable speed motor drives which chop the electrical waveform may be used, which feed large amounts of electrical noise back into the power lines.